The present invention relates to data communications systems such as those used in communicating data over a system utilizing a bidirectional loop as a global highway having nodes for interconnecting to local data highways which in turn interconnect local stations. More particularly this invention relates to methods and means for providing a modem for transmitting and receiving at a local station the signals flowing over the bidirectional loop and the local highway.
In complex data communications systems, such as those utilized in distributed control systems for large industrial plants, there is a necessity for the grouping of stations such as controller stations on local data highways with another global highway interconnecting the local highways in order to minimize the connecting cables and hence the cost of the system. It is also important that the system be as fault tolerant as possible while supporting a large number of stations spread over a large geographical area. One arrangement which has been found to provide fault tolerance under such conditions involves the use of a bidirectional fibre optic loop as the global highway with wire cabling for the local highways each of which connects to the global highway at a node providing the conversion between optical and electrical signals.
The transmission and reception at the local stations in such a system introduces a number of problems, particularly in connection with the bidirectional global highway since each station will normally be receiving two signals from opposite directions at substantially the same time. Thus, the modem which handles the transmission and reception at each of the stations on the local highway must be able to lock onto one of the received signals. In addition, of course, the modem must be able to ignore noise signals and preferably should not include signal mixing, switching, or splitting devices. The present invention provides a novel modem which provides an improved method of coding data for transmission on a complex communications network such as that described above for the purpose of overcoming the problems mentioned above.